Which paraphrase

[Bridget]

Which paraphrase would you use here?

You must be mad to do that.

1. I believe you are mad to do that.
2. It is necessary for you to be mad in order to do that.

[Pete]

Which paraphrase would you use here?

You must be mad to do that.

1. I believe you are mad to do that.
2. It is necessary for you to be mad in order to do that.
When I read "You must be mad to do that" above, I immediately interpreted it to mean your paraphrase 1.

When I read your paraphrase 2, I realized that if the context was right, I could accept paraphrase 2 as a valid meaning of the original. If I were trying to communicate the sense of paraphrase 2 to someone, however, I almost certainly would say, "You have to be mad to do that."

(As I've said before, I am a native speaker of American English; I've lived all my life in Virginia.)

[OddThomas]

Madness as evidenced by the act and madness as a prerequisite to the act are both legitimate readings. This (among other potential situations) is the dilemma of the insanity defense for murder.

The solitary sentence needs context to be definitively interpreted.

[Bridget]

If I were trying to communicate the sense of paraphrase 2 to someone, however, I almost certainly would say, "You have to be mad to do that."

(As I've said before, I am a native speaker of American English; I've lived all my life in Virginia.)

Thanks, Pete. So, in your variety, is have to used more often than must when expressing objective obligation?

[Bridget]

The solitary sentence needs context to be definitively interpreted.

As with "I have three pounds to spend", I imagine. ;)

[Pete]

Thanks, Pete. So, in your variety, is have to used more often than must when expressing objective obligation?
Yes, I'd definitely say that I both hear and say "have to" in that case. I think that most sentences that I hear with "must" express belief/likelihood, though that is purely a subjective feeling on my part.